Tailoring Electrode/Electrolyte Interfaces through Diazonium Chemistry in Aqueous Organic Redox Flow Batteries

Surface engineering of carbon electrodes can play a critical role in mitigating interfacial polarization and improving charge-transfer kinetics in aqueous organic redox flow batteries (AORFBs), yet the mechanistic understanding of electrode/electrolyte interactions remains limited. We demonstrate a rational approach to tune the interfacial electrochemical reactivity of graphite felt electrodes through grafting of diazonium salts bearing negatively charged functional groups. The modified surfaces exhibit enhanced hydrophilicity and increased electrochemical capacitance. The impact of surface charge on electron-transfer behavior was systematically investigated using both negatively and positively charged redox probes, revealing a strong dependence of electrochemical activity on electrostatic interactions. While grafted layers partially hindered the ferro/ferricyanide couple, they maintained the reversibility of the [Ru(NH 3 ) 6 ] 3+ / 2+ system, confirming the charge-selective nature of the modified interfaces. When tested in neutral aqueous electrolytes containing nitroxide-based redox mediators, electrodes functionalized with sulfonate groups exhibited improved redox reversibility and reduced polarization. Flow battery tests using 4-OH-TEMPO electrolytes demonstrated up to 15% greater capacity and reduced polarization losses compared to pristine electrodes, particularly at high current densities. These findings establish diazonium chemistry as a versatile and controllable route to tailor electrode/electrolyte interactions in RFBs.